As I understand what Nathan said when you shoot mirelurk and hit it against Def=1, you roll for a location and if it is the head you got lucky. But when you targeting specifically the head it has Def=2 then you add +1 difficulty for called shot so total Def= 3. If you roll 3 succeses you hit. Then the rule makes sense. But you don’t roll against def=2 when you randomly roll head. As Nathan wrote “The separate defence is for specifically targeting the mirelurk’s head”
Example:
Player: " I shoot mirelurk"
GM: “Great, Def=1”
Player: " I did it. 1 succes".
GM: “Great, you succeded, roll for hit location”
Player: “Head”
GM; “Ahhh you missed, Head has def=2”
So that’s why I think you don’t roll against Def=2 when you randomly hit head